739 research outputs found
On the generation of UHECRs in GRBs: a reappraisal
We re-examine critically the arguments raised against the theory that Ultra
High Energy Cosmic Rays observed at Earth are produced in Gamma Ray Bursts.
These include the limitations to the highest energy attainable by protons
around the bursts' shocks, the spectral slope at the highest energies, the
total energy released in non--thermal particles, the occurrence of doublets and
triplets in the data reported by AGASA. We show that, to within the
uncertainties in our current knowledge of GRBs, none of these objections is
really fatal to the scenario. In particular, we show that the total energy
budget of GRBs easily accounts for the energy injection rate necessary to
account for UHECRs as observed at Earth. We also compute the expected particle
spectrum at Earth, showing that it fits the HiRes and AGASA data to within
statistical uncertainties. We consider the existence of multiplets in AGASA'
data. To this end, we present a Langevin--like treatment for the motion of a
charged particle in the IGM magnetic field, which allows us to estimate both
the average and the rms timedelay for particles of given energy; we discuss
when particles of identical energies reach the Earth in bunches, or spread over
the rms timedelay, showing that multiplets pose no problem for an explosive
model for the sources of UHECRs. We compare our model with a scenario where the
particles are accelerated at internal shocks, underlining differences and
advantages of particle acceleration at external shocks.Comment: Accepted for publication in the Astrophysical Journal; minor change
On gravitomagnetic precession around black holes
We compute exactly the Lense-Thirring precession frequency for point masses
in the Kerr metric, for arbitrary black hole mass and specific angular
momentum. We show that this frequency, for point masses at or close to the
innermost stable orbit, and for holes with moderate to extreme rotation, is
less than, but comparable to the rotation frequency. Thus, if the quasi
periodic oscillations (QPOs) observed in the modulation of the X-ray flux from
some black holes candidates are due to Lense-Thirring precession of orbiting
material, we predict that a separate, distinct QPO ought to be observed in each
object.Comment: Accepted for publication in MNRAS. MN-Latex, 2 figure
Neutral beam model for the anomalous gamma-ray emission component in GRB 941017
Gonz\'alez et al. (2003) have reported the discovery of an anomalous
radiation component from ~ 1 -- 200 MeV in GRB 941017. This component varies
independently of and contains > 3 times the energy found in the prompt ~ 50 keV
-- 1 MeV radiation component that is well described by the relativistic
synchrotron-shock model. Acceleration of hadrons to very high energies can give
rise to two additional emission components, one produced inside the GRB blast
wave and one associated with an escaping beam of ultra-high energy (UHE; >
10^{14} eV) neutrons, gamma rays, and neutrinos. The first component extending
to ~ 100 MeV is from a pair-photon cascade induced by photomeson processes with
the internal synchrotron photons coincident with the prompt radiation. The
outflowing UHE neutral beam can undergo further interactions with external
photons from the backscattered photon field to produce a beam of
hyper-relativistic electrons that lose most of their energy during a fraction
of a gyroperiod in the assumed Gauss-strength magnetic fields of the
circumburst medium. The synchrotron radiation of these electrons has a spectrum
with vF_v index equal to +1 that can explain the anomalous component in GRB
941017. This interpretation of the spectrum of GRB 941017 requires a high
baryon load of the accelerated particles in GRB blast waves. It implies that
most of the radiation associated with the anomalous component is released at >
500 MeV, suitable for observations with GLAST, and with a comparable energy
fluence in ~100 TeV neutrinos that could be detected with a km-scale neutrino
telescope like IceCube.Comment: 4 pages, 1 figure, minor corrections, Astronomy and Astrophysics
Letters, in pres
A Self-Similar Solution for the Propagation of a Relativistic Shock in an Exponential Atmosphere
We derive a fully relativistic, self-similar solution to describe the
propagation of a shock along an exponentially decreasing atmosphere, in the
limit of very large Lorentz factor. We solve the problem in planar symmetry and
compute the acceleration of the shock in terms of the density gradient crossed
during its evolution. We apply our solution to the acceleration of shocks
within the atmosphere of a HyperNova, and show that velocities consistent with
the requirements of GRB models can be achieved with exponential atmospheres
spanning a wide density range.Comment: ApJL in pres
The soft X-ray afterglow of gamma ray bursts, a stringent test for the fireball model
I consider the recent discovery of a soft X-ray source inside the error box
of the gamma ray burst GB 960720 by the SAX, ASCA and ROSAT satellites, in
terms of the fireball model. I show that the ejecta shell, which, after causing
the burst is cold and dense, but still relativistic, keeps plowing through the
interstellar medium, heating up the just-shocked matter which then emits
X-rays. I compute the radiation emitted by this matter. I show that, up to
about two months after the burst, in the cosmological scenario a soft X-ray
(0.1-10 keV) flux of at least 10^(-13) erg / s cm^2, well within current
observational capabilities, is generated, explaining the observations of the
three satellites. Instead, in the Galactic Halo scenario a flux 3 orders of
magnitude lower is expected. Detection of this non-thermal, declining flux in a
statistically significant number of objects would simultaneously establish the
fireball model and the cosmological nature of gamma ray bursts.Comment: Replaces previous version: now it does include figure. Accepted for
publication in the Astrophysical Journal Letter
Ultra high energy neutrinos from gamma ray bursts
Protons accelerated to high energies in the relativistic shocks that generate
gamma ray bursts photoproduce pions, and then neutrinos in situ. I show that
ultra high energy neutrinos (> 10^19 eV) are produced during the burst and the
afterglow. A larger flux, also from bursts, is generated via photoproduction
off CMBR photons in flight but is not correlated with currently observable
bursts, appearing as a bright background. Adiabatic/synchrotron losses from
protons/pions/muons are negligible. Temporal and directional coincidences with
bursts detected by satellites can separate correlated neutrinos from the
background.Comment: Adiabatic/synchrotron losses from protons/pions/muons shown to be
negligible. Accepted for publication in Phys. Rev. Letters. RevTe
On particle acceleration around shocks. I
We derive a relativistically covariant (although not manifestly so) equation
for the distribution function of particles accelerated at shocks, which applies
also to extremely relativistic shocks, and arbitrarily anisotropic particle
distributions. The theory is formulated for arbitrary pitch angle scattering,
and reduces to the well--known case for small angle scatterings via a
Fokker--Planck approximation. The boundary conditions for the problem are
completely reformulated introducing a physically motivated Green's function;
the new formulation allows derivation of the particle spectrum both close and
far away from the injection energy in an exact way, while it can be shown to
reduce to a power--law at large particle energies. The particle spectral index
is also recovered in a novel way. Contact is made with the Newtonian treatment.Comment: Accepted for publication in ApJ; minor changes onl
Gravitational shocks as a key ingredient of Gamma-Ray Bursts
We identify a novel physical mechanism that may be responsible for energy
release in -ray bursts. Radial perturbations in the neutron core,
induced by its collision with collapsing outer layers during the early stages
of supernova explosions, can trigger a gravitational shock, which can readily
eject a small but significant fraction of the collapsing material at
ultra-relativistic speeds. The development of such shocks is a strong-field
effect arising in near-critical collapse in General Relativity and has been
observed in numerical simulations in various contexts, including in particular
radially perturbed neutron star collapse, albeit for a tiny range of initial
conditions. Therefore, this effect can be easily missed in numerical
simulations if the relevant parameter space is not exhaustively investigated.
In the proposed picture, the observed rarity of -ray bursts would be
explained if the relevant conditions for this mechanism appear in only about
one in every core collapse supernovae. We also mention the
possibility that near-critical collapse could play a role in powering the
central engines of Active Galactic Nuclei.Comment: 9 pages, 3 figure
Study of the Correlations Between the Highest Energy Cosmic Ray Showers and Gamma Ray Bursts
We examine the correlation between the arrival direction of ultra high energy
cosmic ray showers and gamma ray bursts in the third BATSE catalog. We find no
correlation between the two data sets. We also find no correlations between a
pre-BATSE burst catalog and the Haverah Park Ultra High Energy shower set that
cover approximately the same period of time.Comment: 1 uuencoded g-zipped postscript file containing text and figure
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